Device with simple DAC?

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Hi there,

I know that this is somewhat off-topic here, but not completely :)

I intend to get some mini-boxes  (http://www.mini-box.com ) for an embedded
Linux Project.

The device gives me all I need except a simple (slow 25HZ) DAC.

All I need is an Output Voltage between 0 and 5 Volts which could be
controlled by means of some simple commands.

Do you have any recommondations for this? Some simple schematics are fine as
well as the alternative would be a generic IO-Device like the Labjack U12
(http://www.labjack.com ).

Regards

Sven

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Re: Device with simple DAC?
 The cheapest way to make a slow, low precision DAC is to RC filter
the output of a PWM (8254?). The second best is an R-2R ladder on a
parallel port. The R-2R is faster, but the resistor drift limits
absolute accuracy to about 8-bits. You'll need to use an op-amp
follower with the ladder, use one with a low input bias current.

On Fri, 20 Feb 2004 14:41:40 +0000 (UTC), Sven Geggus

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Re: Device with simple DAC?

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If you can live with a lower update rate,
One way is to do a PWM output from the parallel port bits and then
RC-filter the outputs. This will give you multiple (12?) possible DACS.
 
The PWM can be done as part of the tic clock interrupt. One trick needed
to get reasonable response time/ripple is to interlace the PWM.

Imagine a 8 bit PWM, one way to do 128 (50% duty cycle) is 128 cycles on
and 128 cycles off but this has a low ripple frequency. A better way is
to do one cycle on and one cycle off for all 256 cycles, thus having a
higher ripple frequency - faster time constant filter - faster response
time...

One simple way to generate interlaced PWM is by bit reversing the PWM
reference count before it is compared with the PWM value...


Peter Wallace

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